Hydraulic power steering apparatus

Abstract: A power steering apparatus comprises a fluid source (10) for discharging pressurized fluid, a power cylinder (12), a control valve responsive to operation of a steering wheel for supplying the power cylinder with pressurized fluid, and a reservoir (11). The control valve has a first control portion for controlling the flow of pressurized fluid from the fluid source to the reservoir, and a second control portion for controlling the flow of pressurized fluid to the power cylinder. The first control portion has four paths communicating with the fluid source and the reservoir, and each of the four paths is formed with a variable orifice (V1-V4) of open-center type which is open when the control valve is in a neutral state. The second control portion has two supply paths communicating with the fluid source and the chambers of the power cylinder, each of the two supply paths being formed with a variable orifice (V5,V6) of semi-open-center type which is open when the control valve is in the neutral state, and two drain paths communicating with the chambers of the power cylinder and the reservoir, each of the two drain paths being formed with a variable orifice (V7,V8) of closed-center type which is closed when the control valve is in the neutral state.

Description: BACKGROUND OF THE INVENTION Field of the Invention : The present invention relates to a hydraulic power steering apparatus suitable for use in automobiles and the like, and more particularly, it relates to the improvement of a hydraulic power steering apparatus described in document EP-A3-0 594 137 which is state of the art according to Art. 54(3) EPC. Discussion of Related Art : In a conventional power steering apparatus, center-open type variable orifices arranged in a rotary control valve operate immediately to supply the pressurized fluid to one of power cylinder chambers and to exhaust the fluid from the other power cylinder chamber even when a steering wheel is turned slightly from the neutral position. This causes the differential pressure across a power cylinder to begin to increase immediately and to more increases gradually as the rotational angle of the rotary control valve further increases. Accordingly, the conventional power steering apparatus does not give the driver a steady steering feeling at around the neutral position. The applicant of this application has proposed in the aforementioned document EP-A3-0 594 137 a power steering apparatus which is designed to increase the rigidity of the steering wheel at around the neutral position by preventing the differential pressure across the power cylinder from increasing when the steering wheel is at around the neutral position. The power steering apparatus as proposed therein is composed of an engine-driven pump 50, a reservoir 51, a power cylinder 52 for assisting the steering operation, and a rotary control valve 53, as shown in FIG. 1. The rotary control valve 53 fluidly controls pressurized fluid which is supplied from the pump 50 to the power cylinder 52, upon rotation of the steering wheel (not shown). The control valve 53 is composed of a first control portion having variable orifices V1-V4 of center-open type (shown in FIG. 4), and a second control portion having variable orifices V5 and V6 of semi-center-open type (shown in FIG 5) communicating to the reservoir 51 and variable orifices V7 and V8 of center-closed type (shown in FIG. 6) communicating to the pump 50. The opening area of each of the variable orifice V1-V8 is varied in correspondence with the relative rotational angle (valve rotational angle) between a valve shaft 61 and a valve body 62, as shown in FIG. 7. In the above-mentioned constitution, since the center-closed variable orifices V7 and V8 are closed in the neutral state of the steering operation, the pressurized fluid is not supplied to the power cylinder 52. In such a state, both of fluid chambers 52A and 52B of the power cylinder 52 are in communication with the reservoir 51 through the semi-center-open variable orifices V5 and V6, so that the pressures in the fluid chambers 52A and 52B are maintained to be extremely low or substantially zero. Accordingly, the pressurized fluid discharged from the pump 50 is drained to the reservoir 51 through the center-open variable orifices V1-V4 of the first control portion. When the valve shaft 61 is rotated relative to the valve body 62 by a slight distance in rightward in FIG. 1 upon rotation of the steering wheel, the opening areas of the variable orifices V2 and V3 of the first control portion are enlarged while those of the variable orifices V1 and V4 thereof are diminished. In such a state, although the pressure of the pump 50 increases gradually, the variable orifices V7 and V8 of the second control portion remains closed until the valve rotational angle reaches 1, so that the differential pressure between the both fluid chambers 52A and 52B of the power cylinder 52 is maintained to be zero. Therefore, within the non-sensitive zone that the valve rotational angle is smaller than 1, the differential pressure between the both fluid chambers 52A and 52B can be prevented from increasing, which provides the same steering feeling as that of a manual steering apparatus, so that the rigidity of the steering wheel at around the neutral position can be increased. As the valve rotational angle exceeds 1, the center-closed variable orifice V8 of the second control portion communicating with the pump 50 begins to open. At this time, since the opening area of the variable orifice V6 communicating with the reservoir 51 is reduced, the pressure in the fluid chamber 52B is increased. With this operation, the differential pressure is generated between the both fluid chambers 52A and 52B to assist the steering operation. When the steering wheel is returned, on the other hand, the rotary control valve 53 comes back to the neutral position. In such a state, a flow passage leading to the both fluid chambers 52A and 52B of the power cylinder 52 is closed by the center-closed variable orifices V7 and V8. Although the supply of the pressurized fluid from the pump 50 to the fluid chamber 52A low in the pressure is possible through the center-open variable orifices V2 and V4 and the semi-center-open variable orifice V5, such supply is delayed due to the resistances of the variable orifices V2, V4 and V5 in the line of the flow passage, which causes the delay in the returning of the steering wheel. SUMMARY OF THE INVENTION A power steering apparatus comprising the features of the pre-characterizing clause of claim 1 is known from document US-A-45 77 660. A primary object of the present invention is to provide an improved power steering apparatus capable of providing a driver with an improved steering feeling. Another object of the present invention is to provide an improved power steering apparatus capable of improving the return of a steering wheel. A further object of the present invention is to provide an improved power steering apparatus capable of reducing energy consumption. According to the invention, these objects are achieved by the power steering apparatus as defined in claim 1. Briefly, the present invention provides a power steering apparatus having a fluid source for discharging pressurized fluid, a power cylinder, a control valve responsive to operation of a steering wheel for supplying the power cylinder with pressurized fluid from the fluid source, and a reservoir. The control valve is composed of first and second control portions. The first control portion has at least one path connecting the fluid source and the reservoir. The path of the first control portion is formed with a variable orifice of center-open type which opens when the control valve is in a neutral state, to control the flow of pressurized fluid from the fluid source to the reservoir. The second control portion has two supply paths making the fluid source communicate respectively with a pair of chambers of the power cylinder and two drain paths respectively making the chambers of the power cylinder communicate with the reservoir, to control the flow of pressurized fluid from the fluid source to the power cylinder. When the control valve is in the neutral state, the two supply paths each formed with a variable orifice of center-open type remain open, while the two drain paths each formed with a variable orifice of center-closed type remain closed. The power steering apparatus according to claim 1 differs from the power steering apparatus according to document EP-A3-0 594 137 in that each of the two supply paths is formed with a variable orifice of center-open type which is open when said control valve is in the neutral state and in that each of the two drain paths is formed with a variable orifice of center-closed type which is closed when the control valve is in the neutral state. In the power steering apparatus according to the invention, the differential pressure across the power cylinder is maintained to be extremely low or substantially zero when the steering wheel is at around the neutral position. Therefore, at around the neutral position of the steering wheel, the steering feeling is the same as that in a manual (or no power assisted) steering apparatus, so that the driver can definitely feel the neutral position of the steering wheel, i.e., the resistance to turn the same is enhanced at around the neutral position. This also enables the driver to feel the beginning of power assist when the steering wheel is turned in either direction beyond a certain valve rotational angle from the neutral position. Further, when the steering wheel is returned with the control valve being in the neutral state, the two drain paths of the second control portion communicating with the reservoir are closed by the center-closed variable orifices. However, at this time, one of the fluid chambers of the power cylinder is in communication with the other thereof through the semi-center-open variable orifices of the second control portion, which permits the fluid to flow between the power cylinder chambers, so that the returning of the steering wheel can be improved. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Various other objects, features and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiments when considered in connection with the accompanying drawings, in which: FIG. 1 is a diagram showing a power steering apparatus proposed in the aforementioned EP-A-594 137, with a rotary control valve enlarged partly; FIG. 2 is a diagram showing a power steering apparatus according to a first embodiment of the present invention; FIG. 3 is a diagram similar to FIG. 1, of the apparatus in the first embodiment shown in FIG. 2; FIG. 4 is an enlarged view of the portion A in FIG. 3; FIG. 5 is an enlarged view of the portion B in FIG. 3; FIG. 6 is an enlarged view of the portion C in FIG. 3; FIG. 7 is a graph showing the relationship between the relative rotational angle of the control valve and the effective opening areas of the variable orifices according to the first embodiment; FIG. 8 is a graph showing the relationship between the relative rotational angle of the control valve and the differential pressure across the power cylinder; FIG. 9 is a diagram similar to FIG. 3, of the apparatus according to a second embodiment; FIG. 10 is an enlarged view of the portion B' in FIG. 9; and FIG. 11 is a graph showing the relationship between the relative rotational angle of the control valve and the effective opening areas of the variable orifices according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 2, a hydraulic power steering apparatus according to the present invention is mainly composed of a pump 10 which is driven by an automotive engine not shown, a reservoir 11, a power cylinder 12 for assisting the steering operation, a rotary control valve 14, and a flow control valve 18. The rotary control valve 14 is responsive to rotation of a steering wheel 13 so as to control the flow of pressurized fluid from the pump 10 to the power cylinder 12. The flow control valve 18 is composed of a valve spool 18a and a spring 20 which is disposed in a spring chamber 20a formed at the back of the valve spool 18a. The flow control valve 18 is disposed in a bypass passage 19, and the flow of fluid from an inlet port 19a to an outlet port 19b of the flow control valve 18 is controlled by the valve spool 18a. A port 20b of the spring chamber 20a is connected to a supply passage 16 via a control orifice 25 and to the reservoir 11 via a relief valve 31. The valve spool 18a is movable in response to the differential pressure across a metering orifice 17 disposed in the supply passage 16 which connects the pump 10 to the rotary control valve 14. Thus, the bypass passage 19 is opened and closed by the valve spool 18a in response to the differential pressure across the metering orifice 17 so as to maintain the flow rate of pressurized fluid supplied to the rotary control valve 14 substantially constant. The valve spool 18a is urged by the spring 20 in a direction to close the bypass passage 19. The rotary control valve 14, shown as an enlarged development view in FIG. 3, is composed of a valve shaft 21 coupled to the steering wheel 13 for rotation therewith, a valve body 22 arranged to surround the valve shaft 21 in a coaxial relationship therewith, a torsion bar (not shown) connecting the valve shaft 21 to the valve body 22 in the manner that they are relatively rotatable within a limited angle, and a valve housing (not shown) receiving the above-mentioned components therein. The valve body 22 is mechanically connected to a steering linkage to which hydraulic force generated by the power cylinder 12 is applied. Each of the outer surface of the valve shaft 21 and the inner surface of the valve body 22 is provided with a plurality of axial slots, which are formed at predetermined intervals in the circumferential direction. Thus, two kinds of control portions, i.e., first control portion 23 and second control portion 24 are formed between the valve shaft 15 and the valve body 16 alternately at intervals of 90 degrees in the circumferential direction so as to control the flow of pressurized fluid. The first control portion 23 is composed of a first bridge circuit C1 having four fluid paths L1, L2, L3 and L4 which are connected to the pump 10 and the reservoir 11, and four variable orifices V1, V2, V3 and V4 are disposed in the middle of the fluid paths L1, L2, L3 and L4, respectively. Each of the variable orifices V1, V2, V3 and V4 is of center-open type, and the initial state (neutral state) of which is shown in FIG. 4. Each of the variable orifices V1, V2, V3 and V4 has such a characteristic that the opening area of each orifice changes as illustrated by the line A1 in FIG. 7 with relative rotation (valve rotational angle) between the valve shaft 21 and valve body 22. The second control portion 24 is composed of a second bridge circuit C2 having four fluid paths L5, L6, L7 and L8 which are connected to the pump 10, both fluid chambers 12A and 12B of the power cylinder 12 and the reservoir 11, and four variable orifices V5, V6, V7 and V8 are disposed in the middle of the fluid paths L5, L6, L7 and L8, respectively. Each of the variable orifices V5 and V6 communicating with the pump 10 is of semi-center-open type, and the initial state of which is shown in FIG. 5. Each of the variable orifices V7 and V8 communicating with the reservoir 11 is of center-closed type, the initial state of which is shown in FIG. 6. Each of the variable orifices V5 and V6 has such a characteristic that the opening area of each orifice changes as illustrated by the line A2 in FIG. 7 with the relative rotation between the valve shaft 21 and valve body 22. Each of the variable orifices V7 and V8 has such a characteristic that the opening area of each orifice changes as illustrated by the line A3 in FIG. 7 with the relative rotation between the valve shaft 21 and valve body 22. In FIG. 2, numeral 30 indicates a supply hose for supplying pressurized fluid to the rotary control valve 14. The operation of the power steering apparatus as constructed above will now be described. When the operation of the pump 10 is initiated, pressurized fluid discharged from the pump 10 to the rotary control valve 14 is controlled by the metering orifice 17 and the flow control valve 18 to substantially constant flow rate and pressurized fluid at such flow rate is supplied to the first and second control portion 23 and 24. When the steering wheel 13 is not rotated, i.e, the rotary control valve 14 is at around the neutral position, the center-closed variable orifices V7 and V8 of the second control portion 24 remain closed. In such a state, since the semi-center-open variable orifices V5 and V6 of the second control portion 24 remain opened, the both fluid chambers 12A and 12B of the power cylinder 12 are in communication with each other through the semi-center-open variable orifice V5 and V6, and the pressurized fluid discharged from the pump 10 is equivalently supplied to the both fluid chambers 12A and 12B. Accordingly, the pressures in the both fluid chambers 12A and 12B are maintained to be substantially same. On the other hand, the pressurized fluid supplied to the first control portion 23 is drained to the reservoir 11 through the center-open variable orifices V1, V2, V3 and V4. When the valve shaft 15 is rotated slightly relative to the valve body 16, for example, in the rightward direction in FIG. 3 upon the steering operation, the opening areas of the variable orifices V2 and V3 of the first control portion 23 increase while the opening areas of the remaining variable orifices V1 and V4 decrease. Thus, the pressure of the fluid discharged from the pump 10, i.e., the supply pressure to the rotary control valve 14 gradually increases. The pressures in the fluid chambers 12A and 12B of the power cylinder 12 are gradually increased, but the differential pressure therebetween is maintained to be zero until the relative rotational angle of the rotary control valve 14 reaches 1, because the center-closed variable orifices V7 and V8 of the second control portion 24 remain in the closed state until the relative rotational angle of the rotary control valve 14 reaches 1. Accordingly, within the neutral zone that the relative rotational angle of the rotary control valve 14 is smaller than 1, the differential pressure between the fluid chambers 12A and 12B of the power cylinder 12 is prevented from increasing, as illustrated by the solid curved line in FIG. 8. Namely, within the neutral zone, the steering feeling is the same as that in a manual (or no power assisted) steering apparatus, and the rigidity against rotation of the steering wheel 13 at around the neutral position can be increased. When the relative rotational angle of the rotary control valve 14 exceeds 1, the semi-center-open variable orifice V5 of the second control portion communicating to the pump 10 begins to close, while the center-closed variable orifice V7 communicating to the reservoir 11 begins to open, whereby the pressurized fluid in the fluid chamber 12A of the power cylinder 12 is drained to the reservoir 11 through the center-closed variable orifice V7. This causes the pressure in the fluid chamber 12A to decrease so as to increase the differential pressure between the both fluid chambers 12A and 12B, whereby the steering operation of the steering wheel 13 can be assisted. At this time, the pressure in the fluid chamber 12A depends on the opening areas of the variable orifices V5 and V7 of the second control portion 24, while that in the fluid chamber 12B depends on the opening areas of the variable orifices V6 of the second control portion 24. When the steering 13 is returned, the position of the valve shaft 21 relative to the valve body 22 comes back to the neutral position by the action of the torsion bar. In this state, since the center-closed variable orifices V7 and V8 are closed, the pressurized fluid in the fluid chamber 12B is flown to the fluid chamber 12A through the semi-center-open variable orifices V6 and V5 when the force returning the steering wheel 13 acts on the piston from the side of the road wheels. This advantageously allows the steering wheel 13 to return smoothly. Since the operation of the rotary control valve 14 when the valve shaft 21 is relatively rotated in the leftward direction as viewed in FIG. 3 is easily understood from the foregoing description, the detail description thereof is omitted for the sake of brevity. As described above, in the power steering apparatus according to the present invention, the differential pressure across the power cylinder 12 is maintained to be extremely low or substantially zero when the steering wheel 13 is at around the neutral position. Therefore, at around the neutral position of the steering wheel 13, the steering feeling is the same as that in the manual steering apparatus, so that the driver can definitely feel the neutral position of the steering wheel, because the rigidity of the steering wheel 13, i.e., the resistance to turn the same is enhanced at around the neutral position. This also enables the driver to feel the beginning of power assist when the steering wheel 13 is turned in either direction beyond a certain angle 1 from the neutral position. Further, in the power steering apparatus according to the present invention, when the steering wheel 13 is returned with the rotary control valve 14 being in the neutral state, the two drain paths L7 and L8 of the second control portion 24 communicating with the reservoir 11 are closed by the center-closed variable orifices V7 and V8, while one of the fluid chambers 12A and 12B of the power cylinder 12 is brought into communication with the other thereof through the semi-center-open variable orifices V5 and V6 of the second control portion 24. Therefore, the pressurized fluid in one of the chambers 12A and 12B is smoothly supplied to the other thereof, so that the returning of the steering wheel 13 can be improved. A second embodiment according to the present invention will now be described with reference to FIGs. 9-11 and FIG. 8. The entire configuration of the system is similar to that shown in FIG. 3. In this second embodiment, as shown in FIG. 9, variable orifices V5 and V6 are formed as of center-open type which have the same configuration as those V1-V4. Namely, each of the variable orifices V5 and V6 has its configuration as shown in FIG. 10. Therefore, as shown in FIG. 11, each of the variable orifices V5 and V6 is closed in the same rate as those V1-V4 as the relative valve rotation advances in either direction. With this configuration, when the steering wheel 13 is turned in either direction, the same steering feeling as in the manual steering apparatus is given to the driver until the relative valve rotation reaches 1 in FIG. 8. However, because the variable orifices V5 and V6 are configured as of center-open type, the pressure increase exerted by the first control portion 23 is immediately applied to the power cylinder 12. This causes the differential pressure across the power cylinder 12 to increase faster than as is done in the first embodiment as the relative valve rotation advances in either direction, as indicated by the phantom curved line in FIG. 8. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.

Claims: A power steering apparatus having a fluid source (10) for supplying pressurized fluid, a power cylinder (12), a control valve (14) responsive to the operation of a steering wheel (13) for supplying said power cylinder (12) with pressurized fluid from said fluid source (10), and a reservoir (11), said control valve (14) comprising: a first control portion (23) for controlling the flow of pressurized fluid from said fluid source (10) to said reservoir (11), said first control portion (23) having at least one path (L1, L3; L2, L4) connecting said fluid source (10) and said reservoir (11); and a second control portion (24) for controlling the flow of pressurized fluid from said fluid source (10) to said power cylinder (12), said second control portion (24) having two supply paths (L5, L6) making said fluid source (10) communicate respectively with a pair of fluid chambers (12A, 12B) of said power cylinder (12) and two drain paths (L7, L8) respectively making the chambers (12A, 12B) of said power cylinder (12) communicate with said reservoir (11), wherein each of said two supply paths (L5, L6) is formed with a variable orifice (V5, V6) of center-open type which is open when said control valve (14) is in the neutral state, characterized in that each of said two drain paths (L7, L8) is formed with a variable orifice (V7, V8) of center-closed type which is closed when said control valve (14) is in the neutral state and in that said at least one path (L1, L3; L2, L4) of said first control portion (23) is formed with at least one variable orifice (V1, V3; V2, V4) of center-open type, said path (L1, L3; L2, L4) being open when said control valve (14) is in the neutral state. A power steering apparatus according to claim 1, characterized in that said fluid chambers (12A, 12B) of said power cylinder (12) communicate with each other through said two supply paths (L5, L6) of said second control portion (24) when said control valve (14) is in the neutral state. A power steering apparatus according to claim 1 or 2, characterized in that each of said variable orifices (V5, V6) formed in said two supply paths (L5, L6) of said second control portion (24) is of semi-center-open type whose opening area is smaller than that of center-open type when said control valve (14) is in the neutral state. A power steering apparatus according to one of Claims 1 to 3, characterized in that said first control portion (23) forms two supply paths (L1, L2) communicating with said fluid source (10) and two drain paths (L3, L4) communicating with said reservoir (11), and in that each of said paths (L1, L2, L3, L4) of said first control portion (23) is formed with a variable orifice (V1, V2, V3, V4) of center-open type which opens when said control valve (14) is in the neutral state. A power steering apparatus according to one of claims 1 to 4, characterized in that said power steering apparatus further comprises: a metering orifice (17) located in a supply passage (16) making said fluid source (11) communicate with said control valve (14); and a flow rate control means (18) operative in response to a differential pressure across said metering orifice (17) so as to maintain a flow rate of pressurized fluid supplied to said control valve (14) from said fluid source (11) substantially constant.

Inventors: HAGA KYOSUKE, MORI KATSUHISA, SUZUKI MIKIO, HAGA, KYOSUKE, MORI, KATSUHISA, SUZUKI, MIKIO
Applicants: TOYODA MACHINE WORKS LTD, TOYODA KOKI KABUSHIKI KAISHA
Application year: 1994
Main classification: B62D5
IPC codes: B62D5